Recently, a secondary battery is a device that stores electrical energy in chemical form and converts the stored chemical energy into electrical energy to generate electricity when needed. The secondary battery is also referred to as a “rechargeable battery” because it can be recharged repeatedly. A common secondary battery includes a lead accumulator, a NiCd battery, a NiMH accumulator, a Li-ion battery, and a Li-ion polymer battery. When compared to a disposable primary battery, not only is the secondary battery more economically efficient, it is also more environmentally friendly.
Secondary batteries are currently used in low power applications, for example, devices for assisting in the starting of car engines, mobile devices, tools, and uninterrupted power supply systems. Recent developments in wireless communication technologies have led to the popularization of mobile devices and have brought about a tendency to connect many types of existing devices to wireless networks. Under such circumstances, demand for secondary batteries is growing explosively. Hybrid vehicle and electric vehicle have been put into practical use in the aspect of environmental pollution prevention. These next-generation automobiles reduce in costs and weight and increase in their life span by employing technologies based on secondary batteries.
Generally, most secondary batteries have a cylindrical, prismatic, or pouch shape. This is because the secondary batteries are fabricated by mounting an electrode assembly composed of a negative electrode, a positive electrode and a separator in a cylindrical or prismatic metal can or a pouch-shaped case of an aluminum laminate sheet, and injecting an electrolyte into the electrode assembly. Thus, because a predetermined mounting space for the secondary battery is required, the cylindrical, prismatic or pouch shape of the secondary battery is a limitation in developing various shapes of mobile devices. Accordingly, there is a need for a new type of secondary battery that is easily adaptable in shape.
To fulfill this need, a linear battery having a very high ratio of length to cross-sectional diameter has been proposed. Korean Patent Publication No. 2005-99903 discloses a flexible battery including an inner electrode, an outer electrode, and an electrolyte layer interposed between the electrodes, but flexibility is not too good. Furthermore, the linear battery uses a polymer electrolyte to form the electrolyte layer, making it difficult for the electrolyte to permeate into an active material of the electrode, resulting in increased battery resistance and reduced capacity and cycle characteristics.
Also, in forming the cable type secondary battery, a non-uniform gap is created between the inner electrode and the outer electrode and the separation layer interposed between the electrodes, and due to this gap, feeding of an electrolyte solution into an outer electrode active material layer is not smooth, causing the battery performance to degrade.
Also, in case that a wire-type current collector is used in the cable type secondary battery, generally because line resistance is higher than sheet resistance, the wire-type current collector has higher resistance characteristics than a sheet-type current collector, causing the battery performance to degrade.